openrisc: Add pci bus support

This patch adds required definitions to allow for PCI buses on OpenRISC.
This is being tested on the OpenRISC QEMU virt platform which is in
development.

OpenRISC does not have IO ports so we keep the definition of
IO_SPACE_LIMIT and PIO_RESERVED to be 0.

Note, since commit 66bcd060 ("parport_pc: Also enable driver for PCI
systems") all platforms that support PCI also need to support parallel
port.  We add a generic header to support compiling parallel port
drivers, though they generally will not work as they require IO ports.

Signed-off-by: Stafford Horne <shorne@gmail.com>